This invention pertains to color separation techniques and structure, for example for use in color projection display systems.
In electronic color projection display systems, the three primary colors, red, green and blue, are generally imaged separately and then combined to produce a color image. One example of a prior art electronic color projection system is shown in "LCD Full-Color Video Projector", Morozumi, Sonchara, Kamakura, Ono, and Aruga, p. 375 SID International Symposium Digest of Technical Papers, 1986. The color of a given portion of the resultant image is determined by the relative intensity of the three primary color beams used to form that portion of the image. It is desired to perform this operation efficiently in order to cause the least light loss.
One prior art color projection display separates white light from a single collimated source into the three primary colors using dichroic beamsplitters having one side exposed to air, which are dielectric interference filters having a sharp response and which does not absorb light, thereby minimizing loss. The three colors are then acted upon independently by three light valves (such as liquid crystal panels available from Seiko-Epson, Japan) and recombined using dichroic beamsplitters to produce the final color image. As shown in the prior art structure of FIG. 1, light from white light source is collimated by lens 2. The collimated white light W is directed to dichroic beamsplitter 3 which reflects the red portion R of white light W to turning mirror 4. The green portion G and blue portion B of white light W pass through beamsplitter 3 and are directed to dichroic beamsplitter 5, which reflects the blue portion B of the green, blue beam G,B toward turning mirror 6. The remaining green portion G of the green, blue beam G,B passes through beamsplitter 5 and passes through light valve 8 to produce green image beam GI, which varies in intensity to provide desired green intensity for the resultant image shown on screen 15 The red portion R of the spectrum is reflected by turning mirror 4 and passes through light valve 10 to produce red image beam RI. Similarly the blue portion B of the spectrum is reflected by turning mirrors 6 and 7 and passes through light valve 9 to produce blue image- beam BI.
The red, green and blue images are formed on light valves 10, 8, and 9, respectively, thereby providing varying attenuation over the image area and over time, resulting in red image beam RI, green image beam GI, and blue image beam BI, respectively. The green image beam GI and blue image beam BI are directed to dichroic beamsplitter 12 for recombination to form a combined green image, blue image beam GI,BI. As shown in FIG. 1, beamsplitter 12, like beamsplitter 5, transmits green light and reflects blue light. This combined beam GI,BI then passes through dichroic beamsplitter 13. Red image beam RI is reflected by turning mirror 11 to dichroic beamsplitter 13 which recombines it with green image, blue image beam GI,BI to form white image beam WI. Like beamsplitter 3, beamsplitter 13 transmits green and blue light, and reflects red light. White image beam WI then passes through lens 14 which projects a superimposed image of red, green, and blue light valves 10, 8 and 9, respectively, onto screen 15.
Since the green portion has passed through four dichroic beamsplitters (3, 5, 12, 13) in succession, the optical path through the system is quite long, and prior art electronic color projectors of this type are much larger than a conventional projector such as movie projectors and slide projectors. Furthermore, the optical path lengths of each of the three colors are different. Since the white light from light source 1 is not perfectly collimated, the fact that the three primary color beams travel different distances means that brightness suffers, as well as color purity, since colors are attenuated to different levels. These have been recognized as serious drawbacks and efforts have been made to reduce the length of the optical path by incorporating crossed beamsplitters as shown in FIG. 2.
The optical path of FIG. 2 is essentially the same as that of FIG. 1, except that beamsplitters 3 and 5 have been incorporated into a single crossed beamsplitter, as have beamsplitters 12 and 13. This has the effect of decreasing the length of the optical path by one half, as well as decreasing the volume required to house the optical devices. Unfortunately, the three primary colors still have different path lengths, as described above with regard to FIG. 1.
Crossed beamsplitters can be fabricated in two ways. The easiest and most straightforward method is to cut one beamsplitter in two and mount the two halves adjacent to the other beamsplitter as shown in FIG. 3. In this case, beamsplitter 13 of FIG. 1 has been cut in two and the two halves mounted adjacent to beamsplitter 12. This is fairly easy to fabricate, although the two halves of beamsplitter 13 must be carefully adjusted to be coplanar. One serious disadvantage of this type of crossed beamsplitter is that a shadow is caused by the crack in the center of beamsplitter 13 which shows up in the center of the projected image. This shadow can be eliminated if beamsplitters 12 and 13 are extremely thin, but this makes it difficult to achieve sufficient beamsplitter flatness to obtain a high quality projected image.
The second method for forming crossed beamsplitters is to construct a cube beamsplitter by coating and cementing four triangular pieces of glass together as shown in FIG. 4. In theory, this eliminates the shadow in the center of the projected image, but such a beamsplitter must be precisely constructed such that:
1. There is no chamfer or chipping at the apex of any of the four triangular glass parts;
2. Each of the coated surfaces is flat all the way to the apex without edge roll;
3. The included angle a is precisely 90 degrees for each of the four glass parts;
4. All dichroic coatings extend, and are uniform, all the way to the apex; and
5. The four glass parts are properly aligned during cementing.
It is difficult to fabricate cube beamsplitters of sufficient quality that no noticeable defect appears in the center of the projected image. Cube beamsplitters are therefore quite expensive.